Most homes in the developed part of the world are connected to a public telephone network through copper wires which lead from the homes to neighboring switching and access equipment, located in a telephone service central office or in a digital loop carrier (DLC) referred to hereinbelow as switching boxes. At the switching boxes, the signals passed on the copper wires are passed onto larger wires (referred to as trunks) which connect to a public telephone network. The usable bandwidth of the copper wires is relatively large (of the order of several MHz), however, regular telephone calls (POTS) utilize only a small portion of the bandwidth.
Standard Voice-band (V series) modems use the bandwidth conventionally used for telephone calls to exchange data packets with an Internet Service Provider (ISP). In most cases clients connect to the ISP immediately before they need to connect to the Internet and disconnect immediately thereafter, generally because the costs of telephone calls are a function of their duration. Establishing the modem connection requires between about 0.5–1 minutes during which the client cannot use the connection. Clients who pay for telephone calls according to their duration must pay also for the time in which the modem connection is being established.
Digital subscriber line (DSL) connections provide continuous communication with the ISP at a flat rate, such that there is no need to establish a connection each time access to the Internet is sought. In addition, DSL connections provide larger bandwidth connections as they utilize nearly the entire usable bandwidth of the copper wires. In forming a DSL connection, a pair of DSL modems are located on the opposite ends of the copper wires, in the user's home and in a digital subscriber line access multiplexer (DSLAM) in (or near) the central office or DLC. One or more links, usually separate from the telephone trunks, connect the DSLAM to a data network. In many cases the bandwidth of the link connecting the DSLAM to the data network is smaller than required to handle data from all the client modems connected to the DSLAM. The size of the link is chosen based on statistical measurements and the use of the link is distributed between the client DSL modems based on quality of service (QoS) and other criterions.
When there is interference on the copper wires, for example when the copper wires are used concurrently with the DSL connection for a telephone call, the data rate of the DSL connection is reduced to allow proper detection of the signals despite the interference. The DSL modems may adjust to the lower transmission rate using a procedure referred to as fast retraining or retrain, for example, as described in the G.992.1 and G.992.2 ITU recommendations.
The number of DSL modems positioned in a DSLAM is limited by various physical constrains, such as the space required for the DSL modems, their power consumption, and their heat dissipation capabilities (referred to as maximum heat density).
One method used to reduce the maximum heat density is using single-chip modem pools which serve a plurality of client DSL modems in parallel. These DSL modem pools have sufficient processing power to serve the connections to all the client modems they serve at the maximal usable bandwidth of the twin pair copper wires. One such modem pool is the TNETD4000C ADSL central office chip-set of Texas Instruments, Inc.